Out of concern for the global environment, there has been a demand in recent years that steel structural members for automobiles have a light weight and a high strength. Techniques for increasing impact energy absorbing ability are also being developed in order to increase the safety of automobile bodies in collisions.
For example, metal tubes such as steel tubes are used as beams for reinforcing doors in order to increase safety during side impacts. The impact energy absorbing ability of door reinforcing beams is increased by giving a metal tube a bent shape. The impact energy absorbing ability of a center pillar reinforcing member is improved by optimizing the shape and curvature of the reinforcing member. Various techniques are being employed for bending steel tubes (in this description, steel tubes include not only usual steel tubes such as welded steel tubes but also members produced by forming a steel sheet into a tubular shape) into an optimal shape for automotive parts.
There is a strong demand for an increase in the tensile strength of automotive parts in order to decrease the weight of automotive bodies. High-strength steel materials having a high strength level such as, for example, a tensile strength of at least 780 MPa or even at least 900 MPa, which is completely different from that used in the past, are being widely used.
It is difficult to carry out bending of a steel material made from a high tensile strength steel in a cold state. Furthermore, bending of a steel material made from a high tensile strength steel in a hot state is also difficult because non-uniform strains develop in the steel material, which causes the shape of a bent product to fluctuate, and shape retention is inadequate. Furthermore, there is a strong demand for the development of bending techniques which can manufacture with high accuracy a steel member having various bent shapes such as a bent shape in which the bending direction varies two-dimensionally or three-dimensionally.
In PCT/JP2006/303220, the present applicant disclosed an apparatus for manufacturing a bent member. That apparatus (a) supports a material being worked at two locations spaced from each other in the axial direction of the material so that the material can move in its axial direction, (b) it feeds the material being worked in the axial direction of the material, (c) it rapidly heats the material being worked with an induction heating coil between the two locations where the material being worked is supported to a temperature at which plastic working of the material is possible and at which the crystal grains of the material do not coarsen, (d) it cools the heated material being worked with a cooling device, and (e) it two-dimensionally or three-dimensionally varies the position of a movable roller die which supports the material being worked at the downstream in the feed direction of the material being worked of the two locations where the material being worked is supported whereby a bending moment is imparted to the high temperature portion of the material being worked and this portion is plastically deformed.
It is economical to carry out induction heating of the material being worked by that apparatus in air. Steel members used in automotive parts basically undergo chemical conversion treatment and electrodeposition coating. In order to increase corrosion resistance, automobile parts are made from a zinc-based coated steel material.
Accordingly, by using zinc-based coated steel as the material being worked by that apparatus, it is not only possible to prevent oxidation of the material being worked but it is also possible to manufacture a bent member or a quench hardened member having excellent corrosion resistance. Therefore, it is possible to widely employ a material which has been worked by that apparatus in automobile parts.
However, when a zinc-based coated steel material is heated to at least the A1 transformation point or even to at least the A3 transformation point, the performance of the coating layer may deteriorate. This is because the vapor pressure of Zn suddenly rises with an increase in temperature, as exemplified by the fact that the vapor pressure is 200 mm Hg at 788° C. and 400 mm Hg at 844° C. As a result, Zn may vaporize during a heating step.
A manufacturing method for a member which is strengthened by high frequency quench hardening is disclosed in Patent Document 1 as a technique for heat treatment of a zinc-based coated steel sheet. In that method, a galvanized steel sheet for high frequency hardening is heated to a quench hardening temperature of at least the Ar3 point and at most 1000° C. and then cooled with the interval from the start of heating until cooling to 350° C. being restricted to at most 60 seconds. Patent Document 1 discloses that even if a hot-dip zinc-based coated steel sheet having a base steel sheet for quench hardening is used as a material to be strengthened by high frequency hardening and high frequency quench hardening is carried out on a portion which is to be increased in strength, a coating layer remains on the quench hardened portion, and the Fe concentration in the coating layer can be restricted to at most 35% (in this description, unless otherwise specified, % means mass %). Therefore, a member for automobiles having excellent coatability and corrosion resistance is obtained.
Patent Document 2 and Patent Document 3 disclose methods in which a material being worked made of a zinc-based coated steel sheet is heated for several minutes at 700-1000° C., for example, and hot pressing is carried out.
Prior Art DocumentsPatent DocumentsPatent Document 1:JP 2000-248338 APatent Document 2:JP 2001-353548 APatent Document 3:JP 2003-73774 A